Apodization of refractive index profile in volume gratings

ABSTRACT

A grating coupler may be fabricated by exposing a photopolymer layer to grating forming light for forming periodic refractive index variations in the photopolymer layer. The photopolymer layer may be exposed to apodization light for reducing an amplitude of the periodic refractive index variations in a spatially-selective manner. The apodization may also be achieved or facilitated by subjecting outer surface(s) of the photopolymer layer to a chemically reactive agent that causes the refractive index contrast to be reduced near the surface(s) of application. The apodized refractive index profile of the gratings facilitates the reduction of optical crosstalk between different gratings of the grating coupler.

REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.17/176,964 entitled “APODIZATION OF REFRACTIVE INDEX PROFILE IN VOLUMEGRATINGS”, filed Feb. 16, 2021, which claims priority from U.S.Provisional Application No. 63/092,288 filed on Oct. 15, 2020 andentitled “Photoinduced Apodization of Refractive Index Profile in VolumeBragg Gratings”, and from U.S. Provisional Application No. 63/114,226filed on Nov. 16, 2020 and entitled “Chemical Diffusion Treated VolumeHolograms and Methods for Making the Same”, all of which beingincorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to optical devices, and in particular tooptical gratings and lightguides using optical gratings.

BACKGROUND

An artificial reality system may include a near-eye display (e.g., aheadset or a pair of glasses) configured to present content to a user.The near-eye display may display virtual objects or combine images ofreal objects with virtual objects, as in virtual reality (VR), augmentedreality (AR), or mixed reality (MR) applications. For example, in an ARsystem, a user may view both images of virtual objects (e.g.,computer-generated images (CGIs)) and the surrounding environment byseeing through a “combiner” component, which is a physical structurewhere display light and environmental light merge as one within theuser's field of view. The combiner of a wearable heads-up display istypically transparent to environmental light but includes some lightrouting optic to direct the display light into the user's field of view.

Wearable heads-up displays may employ lightguides as transparent ortranslucent combiners. Lightguides typically consist of plates of atransparent material with a higher refractive index then the surroundingmedium, usually air. Light input into the plate propagates along thelength of the plate as long as the light continues to be incident atboundaries between the plate and the surrounding medium at an angleabove the critical angle. Lightguides employ in-coupling andout-coupling elements to ensure that light follows a specific path alongthe waveguide and then exits the waveguide at specific location(s) tocreate an image visible to the user. The in-coupling and out-couplingelements need to accurately convey the angular distribution ofbrightness of the in-coupled light beam to the user's eyes to preventdistortion and splitting of the displayed images.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIGS. 1A and 1B are side cross-sectional views of pupil-replicatinglightguides having input and output volume gratings, thepupil-replicating lightguide of FIG. 1A being configured to provide adifferent portion of field of view (FOV) than the pupil-replicatinglightguide of FIG. 1B;

FIG. 2 is a side cross-sectional views of a pupil-replicating lightguidehaving multiplexed volume gratings of FIGS. 1A and 1B for a broaderoverall FOV;

FIG. 3A is an angular dependence of diffraction wavelengths of aplurality of sparsely spaced volume gratings usable in thepupil-replicating lightguide of FIG. 2 ;

FIG. 3B is an angular dependence of diffraction efficiency of theplurality of volume gratings of FIG. 3A;

FIG. 3C is an angular dependence of diffraction wavelengths of aplurality of densely spaced volume gratings usable in thepupil-replicating lightguide of FIG. 2 ;

FIG. 3D is an angular dependence of diffraction efficiency of theplurality of volume gratings of FIG. 3C;

FIG. 3E is a magnified view of the angular dependence of FIG. 3Csuperimposed with local diffraction efficiency plots for two neighboringvolume gratings;

FIG. 3F is a side cross-sectional view of a pupil-replicating lightguideillustrating optical crosstalk;

FIG. 4A is an angular reflectivity plot of two non-apodized volumegratings in the pupil-replicating lightguide of FIG. 2 ;

FIG. 4B is an angular reflectivity plot of two apodized volume gratingsin the pupil-replicating lightguide of FIG. 2 ;

FIG. 4C is a combined angular reflectivity plot of volume gratings withdifferent refractive index contrast Δn;

FIG. 5 is a three-dimensional view of an optical coupler including aphotopolymer layer and a volume grating written in the photopolymerlayer;

FIG. 6 shows example apodization profiles of volume gratings in theoptical coupler of FIG. 5 ;

FIG. 7A is a schematic diagram of exposing a photopolymer layer withapodization light and grating forming light where the apodizationexposure precedes the grating forming exposure;

FIG. 7B is a schematic diagram of exposing a photopolymer layer withapodization light and grating forming light where the apodizationexposure and the grating forming exposure are performed concurrently;

FIG. 7C is a schematic diagram of exposing a photopolymer layer withapodization light and grating forming light where the apodizationexposure is performed after the grating forming exposure;

FIG. 8 is a chart showing a relationship between various embodiments ofa method for providing volume grating refractive index contrastapodization in accordance with this disclosure;

FIG. 9 is a schematic diagram of exposing a photopolymer layersandwiched between layers facilitating a chemically induced apodization;

FIG. 10 is an example chemical structure of the photopolymer layer ofFIG. 9 ;

FIG. 11 is a flow chart of a method of fabrication of a grating couplerof this disclosure; and

FIG. 12 is schematic a view of an augmented reality (AR) display of thisdisclosure having a form factor of a pair of eyeglasses.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated.

Lightguides are used in optical devices to carry light from one locationto another. Pupil-replicating lightguides are used in near-eye displaysfor providing multiple laterally offset copies of a fan of light beamscarrying an image in angular domain for observation by a user of anear-eye display. The multiple offset copies of the beam fan are spreadover an eyebox of the display, making observation of the image lessdependent on the eye position in the eyebox.

Pupil-replicating lightguides may use diffraction gratings forin-coupling and out-coupling image light. Volume gratings, such asvolume Bragg gratings (VBGs) or volume holograms, for example, canin-couple and out-couple image light with high efficiency. VBGs howevertypically operate in a rather narrow angular range. To increase theoverall angular range, multiple pairs of in-coupling and out-couplingVBGs may be provided in a pupil-replicating lightguide. VBGs ofdifferent pairs may have optical crosstalk. When the image light isreflected by an out-coupling VBG after being in-coupled by anin-coupling VBG of a different VBGs pair, a ghost image may appear.

In accordance with this disclosure, optical crosstalk and resultingimage ghosting and contrast/clarity reduction may be lessened byapodizing the refractive index profile of volume gratings in a directionof thickness of the pupil-replicating lightguide. Such apodization maybe achieved chemically and/or photochemically. In a photochemicalapodization process, one or both surfaces of a photopolymer layer areexposed to apodization light for reducing an amplitude of the periodicrefractive index variations. Since most light is present proximate tothe surface(s) being illuminated, the grating gets apodized in thedirection of grating thickness. In a chemical apodization process, oneor both surfaces of the photopolymer layer are exposed to a reactantthat reduces an amplitude of the periodic refractive index variationsnear the one or both surfaces. In some embodiments, both the chemicaland photochemical apodization processes may be used to achieve therequired grating apodization profiles.

In accordance with this disclosure, there is provided a method offabrication of a grating coupler. The method comprises exposing aphotopolymer layer having a thickness between opposed first and secondsurfaces to grating forming light for forming periodic refractive indexvariations in the photopolymer layer. The first surface of thephotopolymer layer is exposed to apodization light for reducing anamplitude of the periodic refractive index variations proximate thefirst surface. The method may include exposing the second surface of thephotopolymer layer to apodization light for reducing an amplitude of theperiodic refractive index variations proximate the second surface. Thefirst surface of the photopolymer layer may be exposed to theapodization light before the photopolymer layer is exposed to thegrating forming light.

In embodiments where a wavelength of the grating forming light isdifferent from a wavelength of the apodization light, the apodizationlight may be absorbed in the photopolymer layer stronger than thegrating forming light. For example, a transmittance of the photopolymerlayer at the wavelength of the apodization light may no greater than 5%.The periodic refractive index variations in the photopolymer layer maybe formed due to a photoreactive agent of the photopolymer layer beingsensitive to illumination with the grating forming light. The amplitudeof the periodic refractive index variations may be reduced due to thephotoreactive agent being sensitive to illumination with the apodizationlight. A duration of the exposure of the photopolymer layer to theapodization light may be shorter than a duration of the exposure of thephotopolymer layer to the grating forming light.

In some embodiments, the exposure of the photopolymer layer to theapodization light is performed concurrently with the exposure of thephotopolymer layer to the grating forming light. The photopolymer layermay include a photoreactive agent for forming the periodic refractiveindex variations by a photoreaction to the grating forming light, and aphotoinhibitor agent for impeding the photoreaction when illuminatedwith the apodization light. A wavelength of the grating forming lightmay be different from a wavelength of the apodization light. The gratingforming light may substantially not activate the photoinhibitor agent,and the apodization light may substantially not activate thephotoreactive agent. The photoreaction may include photopolymerization,and the photoinhibitor agent may undergo photolysis when illuminatedwith the apodization light to produce a radical for impeding thephotopolymerization. The photoinhibitor agent may include e.g. at leastone of butyl nitrite, hexaarylbiimidazole, or tetraethylthiuramdisulfide.

In some embodiments, the exposure of the photopolymer layer to theapodization light is performed after the exposure of the photopolymerlayer to the grating forming light. The photopolymer layer may include aphotoreactive group that reduces the amplitude of the periodicrefractive index variations upon illumination with the apodization lightby at least one of photoisomerization, photoelimination,photopolymerization, or photolocking. For example, photoreactive groupmay include at least one of azobenzene, stilbene, spiropyran,diarylethene, a diazo group, or an azido group. The photoreactive groupmay be on a polymer backbone of the photopolymer layer.

In accordance with this disclosure, there is provided a method offabrication of a grating coupler. The method includes exposing aphotopolymer layer having a thickness between opposed first and secondsurfaces to grating forming light for forming periodic refractive indexvariations in the photopolymer layer, and exposing at least the firstsurface of the photopolymer layer to a reactive agent for reducing anamplitude of the periodic refractive index variations proximate thefirst surface.

In embodiments where a photopolymer of the photopolymer layer comprisesa photopolymerizable group connected to an end group by anacid-cleavable linker group, and where a local refractive index isdefined, at least in part, by the end group, the reactive agent mayinclude an acid for separating the end group from the photopolymerizablegroup. In operation, the end group diffuses away upon being separatedfrom the corresponding photopolymerizable group by application of theacid to the at least first surface, thereby reducing the amplitude ofthe periodic refractive index variations proximate the at least firstsurface.

In accordance with this disclosure, there is further provided a gratingcoupler for a waveguide. The grating coupler includes a photopolymerlayer having a thickness between opposed first and second surfaces, thephotopolymer layer comprising periodic refractive index variations dueto exposure to grating forming light. An amplitude of the periodicrefractive index variations proximate the first surface and/or thesecond surface may be reduced by at least one of: exposing the firstsurface the photopolymer layer to apodization light; or exposing thefirst surface the photopolymer layer to a reactive agent. The periodicrefractive index variations in the photopolymer layer may be formed dueto a photoreactive agent of the photopolymer layer being sensitive toillumination with the grating forming light. The amplitude of theperiodic refractive index variations may be reduced due to thephotoreactive agent being sensitive to illumination with the apodizationlight.

Examples of lightguides with apodized gratings will now be presented.Referring first to FIG. 1A, a pupil-replicating lightguide 100A includesa substrate 110 and an in-coupling grating 102A in the substrate 110 forin-coupling image light 104A into the pupil-replicating lightguide 100A,and an out-coupling grating 106A in the substrate 110 for out-couplingportions 108A of the image light 104A along a length direction of thepupil-replicating lightguide 100A (X-direction in FIG. 1A). The imagelight 104A propagates in the substrate by series of reflections fromopposed top 121 and bottom 122 surfaces of the substrate 110. The imagelight 104A carries a portion of an image in angular domain,corresponding to a narrow cone of rays oriented approximatelyperpendicular to the pupil-replicating lightguide 100A, as shown in FIG.1A. In this example, the in-coupling grating 102A and the out-couplinggrating 106A have a same pitch, such that the out-coupled portions 108Aretain the beam angles of the impinging image light 104A.

Referring to FIG. 1B, a pupil-replicating lightguide 100B is similar tothe pupil-replicating lightguide 100A of FIG. 1A. The pupil-replicatinglightguide 100B of FIG. 1B includes an in-coupling grating 102B in thesubstrate 110 for in-coupling image light 104B and an out-couplinggrating 106B in the substrate 110 for out-coupling portions 108B of theimage light 104A along a length direction of the pupil-replicatinglightguide 100B (X-direction in FIG. 1B). The image light 104Bpropagates in the substrate by series of reflections from opposed top121 and bottom 122 surfaces of the substrate 110. The image light 104Bcarries a different portion of the image in angular domain,corresponding to a narrow cone of rays oriented at an acute, i.e.non-perpendicular, angle to the pupil-replicating lightguide 100B. Thein-coupling grating 102B and the out-coupling grating 106B have a samepitch, such that the out-coupled portions 108B retain the beam angles ofthe impinging image light 104B.

Referring now to FIG. 2 , a pupil-replicating lightguide 200 includes anin-coupler 202 in a substrate 210. The in-coupler 202 includes aplurality of multiplexed in-coupling gratings, e.g. the in-couplinggrating 102A of FIG. 1A, the in-coupling grating 102 b of FIG. 1B, andother in-coupling gratings of different pitches or periods, superimposedin the pupil-replicating lightguide 200. The volume gratings may occupya same volume area of the substrate 210, and/or may be disposed atdifferent depths in the substrate 210. Different gratings of theplurality of in-coupling gratings are configured to in-couple the imagelight 204 impinging onto the substrate 210 at different angles ofincidence. Together, the in-coupling gratings in-couple image light 204covering an entire field of view (FOV) of an image in angular domain tobe carried by the pupil-replicating lightguide 200 and displayed to auser. The in-coupled image light 204 propagates in the substrate byseries of reflections from opposed top 221 and bottom 222 surfaces ofthe substrate 210.

An out-coupler 206 in the substrate 210 includes a plurality ofmultiplexed out-coupling gratings, e.g. the out-coupling grating 106A ofFIG. 1A, the out-coupling grating 106 b of FIG. 1B, and otherout-coupling gratings, superimposed in the pupil-replicating lightguide200. Different gratings of the plurality of out-coupling gratings areconfigured to out-couple the image light 204 propagating in thesubstrate 210 at different angles of diffraction. Together, theout-coupling volume gratings out-couple portions 208 of the image light204 covering the entire FOV. Different portions of the FOV are beingconveyed by the pupil-replicating lightguide 200 by different matchingpairs of gratings. It is further noted that one out-coupling grating peran in-coupling grating is only meant as an example. Two or moreout-coupling gratings may be provided per each in-coupling grating.Various lightguide types, including straight lightguides, curvedlightguides,1D/2D lightguides, etc., may be configured to have matchinggrating pairs.

For the pupil-replicating lightguide 200 to operate as intended, theimage light 204 portions should be redirected only by gratings of a samein-coupling and out-coupling volume grating pair, corresponding to asame particular FOV portion. If a portion of the image light 204 isin-coupled into the pupil-replicating lightguide 200 by a volume gratingfrom one in-coupling/out-coupling volume grating pair, and isout-coupled by a grating from another in-coupling/out-coupling volumegrating pair, an offset image (ghosting) will result.

Origins of the optical crosstalk between different volume grating pairsare further illustrated in FIGS. 3A to 3F. Referring first to FIGS. 3Aand 3B, a grating coupler may include a plurality of volume gratingshaving angular dependencies 312 of diffraction wavelength λ (FIG. 3A)offset relative to one another along an axis of the diffraction angle Θ,due to the volume gratings having different pitches. The angulardependencies 312 are shown for a bandwidth 314 of illuminating light.The angular dependencies 312 are sparsely spaced in the diffractionangle Θ in this example, which results in gaps 315 between plots 316 ofangular dependence of diffraction efficiency η of the individual volumegratings (FIG. 3B). When such grating coupler is used in apupil-replicating lightguide, the gaps 315 will result in FOV gaps inthe displayed image.

The gaps 315 may be avoided by providing tighter spacing between thepitch values of the gratings multiplexed in a grating coupler, whichwill cause the angular dependencies 312 to be spaced closer together.Referring to FIGS. 3C and 3D, the angular dependencies 312 are denselyspaced in angle (FIG. 3C), eliminating gaps between angular efficiencyplots 316. The angular efficiency plots 316 “coalesce” in a continuous,gap-free angular efficiency curve 317 (FIG. 3D). The grating couplerwith the densely spaced (in angular domain) volume gratings will resultin a continuous, gap-free FOV.

Too close a spacing of the grating pitches and resulting gap-free FOVmay result in optical crosstalk, which manifests itself in imagecontrast loss and/or the appearance of ghost images. Referring to FIG.3E for example, angular dependencies 312, 312* of first and seconddiffraction wavelengths of neighboring gratings (i.e. neighboring inpitch) are shown superimposed with corresponding magnified first andsecond diffraction efficiency curves 316, 316* for these gratings. Whenthe first and second diffraction efficiency curves 316, 316* aredisposed too close to each other, crosstalk may result causing the lightto be diffracted by a “wrong” grating.

The latter point is illustrated in FIG. 3F showing a pupil-replicatinglightguide 300. A light beam 304 impinges onto an in-coupler comprisinga plurality of in-coupling gratings, including a first in-couplingvolume grating 302, in a substrate 310. The other in-coupling gratingsare not shown for clarity. The in-coupling grating 302 redirects thelight beam 304 to propagate in the pupil-replicating lightguide 300towards an out-coupler comprising a plurality of out-coupling gratingsin the substrate 310. The out-coupler includes a first out-couplinggrating 306 matching the first in-coupling grating 302 and having thefirst angular dependence 316 (FIG. 3E) of diffraction efficiency η, anda second out-coupling grating 306* (FIG. 3F) having the second angulardependence 316* of diffraction efficiency η. Only two out-couplinggratings are shown in FIG. 3F for clarity. The fringes of thein-coupling and out-coupling gratings may form an acute angle with thesubstrate 310, as shown in FIG. 3F.

In operation, a first output light beam 308 diffracts from a “correct”,i.e. the matching first out-coupling grating 306. A second output lightbeam 308* diffracts from a “wrong” grating, i.e. the second out-couplinggrating 306*. The second output light beam 308* propagates in adifferent direction than the first output light beam 308 because thesecond out-coupling grating has a slightly different pitch than thefirst out-coupling grating. The second output light beam 308* carries anincorrect image, i.e. a ghost image.

The origins of the “incorrect” reflection are further illustrated inFIG. 4A, where reflectivity R of two adjacent volume gratings is plottedagainst an angle of reflective diffraction θ in degrees. Thereflectivity R corresponds to the diffraction efficiency η in areflective volume grating configuration. For each grating, thereflectivity dependence on the angle of diffraction R(θ) includes acentral peak 401A and sidelobes 402A on both sides of the central peak401A. It is seen that the sidelobes 402A of one grating may overlap withan area of the central peak 401A of the other grating. The overlap meansthat, while most of the image light is reflected by the centralreflectivity peak 401A of the “correct” grating as the first outputlight beam 308 (FIG. 3F), a small portion of the image light may bereflected by a sidelobe of the “incorrect” grating producing the secondoutput light beam 308*. It is the diffraction on the “incorrect” outputgratings that causes the contrast loss/image ghosting to occur.Therefore, the sidelobes 402A of the reflectivity dependences R(θ) areundesirable, because they degrade the image quality.

In accordance with this disclosure, sidelobes of an angular reflectivityplot of a volume grating and associated image ghosting may be suppressedby apodizing the grating along a thickness dimension of the substratehosting the grating, i.e. generally in a direction substantiallyperpendicular to a pitch direction of the array of fringes of thegrating. In FIG. 4B, a reflectivity R of apodized gratings from twoadjacent grating pairs is plotted against an angle of reflectivediffraction θ in degrees. Only central peaks 401B are present, withoutany sidelobes. Accordingly, the image light may not reflect from an“incorrect” grating of a grating pair, resulting in a ghost-free image.At least, image ghosts may be considerably suppressed. It is noted thatgratings in grating couplers considered herein may also operate intransmission instead of reflection.

Referring to FIG. 4C, an angular dependence of reflectivity R of avolume grating is plotted for non-apodized gratings with differentgrating strength, i.e. with different amplitude of the refractive indexvariation Δn. For an optimal performance, an apodized volume gratingshould have as high as possible refractive index contrast in a centralpeak area 411, and as low as possible refractive index contrast insidelobe areas 412.

Referring now to FIG. 5 , an optical coupler 500 may be a part of thepupil-replicating lightguide 100A of FIG. 1A, the pupil-replicatinglightguide 100B of FIG. 1B, the pupil-replicating lightguide 200 of FIG.2 , and the pupil-replicating lightguide 300 of FIG. 3F. The opticalcoupler 500 includes a photopolymer (PP) layer 510 having opposed first521 and second 522 surfaces parallel to XY plane. The direction ofthickness t of the substrate is Z-direction in FIG. 5 . The PP layer 510may, but does not have to be, flat. The optical coupler 500 representsan in-coupler for in-coupling an impinging light beam, as well as anout-coupler for out-coupling portions of the light beam at differentlocations along the PP layer 510. The optical coupler 500 may include aplurality of volume gratings 520, e.g. at least 10, 20, 50, 100, or morevolume gratings having different grating pitches, written in thephotopolymer material of the PP layer 510.

The volume gratings 520 may be formed by exposing the PP layer 510 tograting forming light e.g. an interference pattern of two coherent lightbeams illuminating opposed top 521 and bottom 522 surfaces with obliquecoherent beams light. Other configurations, e.g. non-oblique beams,non-opposing beams, are also possible. The photopolymer material of thePP layer 510 changes its refractive index in areas of high intensity ofthe grating forming light, while in areas of low intensity of thegrating forming light the refractive index remains unchanged, or changesvery little.

Turning to FIG. 6 with further reference to FIG. 5 , a profile 601 of arefractive index contrast, that is, a difference between a refractiveindex of the volume grating fringes and a refractive index of the PPlayer 510, is plotted as a function of the thickness coordinate Z. Thefirst profile 601 is described by a Gaussian function in this example.Non-Gaussian function may also be used. More generally, the refractiveindex contrast variation of each volume grating of the plurality ofvolume gratings may have a bell-shaped or a similar function. Thebell-shaped function may have the tip of the bell inside the PP layer510 and the lip of the bell at outer surfaces of the PP layer 510. Inother words, the bell-shaped function may monotonically increase towardsa center thickness of the PP layer 510 from both outer surfaces (i.e.top and bottom surfaces in FIG. 5 ) of the PP layer 510. The maximummay, but does not have to, be disposed proximate a middle of thethickness t of the PP layer 510. For example, in some embodiments thebell-shaped profile may be skewed toward one side, such as a secondprofile 602. The fringes of the grating couplers disposed in the PPlayer 510 may form an acute angle with the PP layer 510, while theirrefractive index contrast varies according to the first 601 or second602 profiles. A uniform profile 603, i.e. that of a non-apodizedgrating, is also shown for a comparison. Different volume gratings mayspatially overlap in the PP layer 510 while having a same or a differentz-profile of the refractive index contrast.

The grating apodization may be achieved by illuminating at least one ofthe top 521 and bottom surfaces 522 of the PP layer 510 with apodizationlight beams, which may be oriented e.g. along Z-direction. A wavelengthor wavelengths of the apodization light may be selected such that atleast a major portion of the apodization light is absorbed beforereaching the middle of the photopolymer layer. The illumination of thePP layer 510 with the apodization light may facilitate the reduction ofthe refractive index contrast near the top 521 and bottom 522 surfacesof the PP layer 510, which causes the grating to be apodized.

Referring now to FIGS. 7A, 7B, and 7C, example exposure sequences andgeometries of the photopolymer layer or film are presented as anon-limiting illustration. In FIG. 7A, the PP layer 510 is first exposedto apodization light 702, and is then exposed to grating forming light704. The apodization light 702 at a wavelength λ_(apodization) mayuniformly illuminate the PP layer 510 from top and/or bottom, withilluminating light beams oriented along Z-direction. The illuminatingbeams may be coherent or non-coherent.

The grating forming light 704 at a grating forming wavelengthλ_(grating) may include coherent first 711 and second 712 collimatedlight beams illuminating the PP layer 510 at an acute angle, such thatthe interference pattern formed by the first 711 and second 712 beamsincludes a periodic pattern of high and low intensity areas (i.e.optical interference fringes) in the PP layer 510. A variety of gratingconfigurations, e.g. ones with curved, tilted grating fringes, 2Dgrating, etc., or any other grating configuration may be provided.

FIG. 7B corresponds to a situation where the PP layer 510 isconcurrently exposed to the apodization light 702 and the gratingforming light 704.

FIG. 7C illustrates a situation where the PP layer 510 is first exposedto grating forming light 704, and then is exposed to the apodizationlight 702.

In the embodiments of FIGS. 7A, 7B, and 7C, the exposure of the PP layer510 to the apodization light 702 may be single-sided, such that only oneof the top 521 or bottom 522 surfaces (FIG. 5 ) are exposed. Theexposure may also be double-sided, such that both the top 521 and bottom522 surfaces are exposed. The top 521 and bottom 522 surfaces may beexposed simultaneously or sequentially, i.e. one after another. Inembodiments where the wavelength λ_(apodization) of the apodizationlight 702 is different from the grating forming wavelength λ_(grating)of the grating forming light 704, the apodization light 702 may beabsorbed by the PP layer 510 more strongly than the grating forminglight 704. Strong absorption of the apodization light 702 causes therefractive index contrast of the volume gratings 520 to be reduced atthe top 521 and bottom 522 surfaces as compared to the refractive indexcontrast within the PP layer 510, e.g. at a middle thickness of the PPlayer 510. A duration of exposure of the PP layer 510 to the apodizationlight 702 may be shorter than a duration of exposure of the PP layer 510to the grating forming light 704.

Several non-limiting embodiments of a method of forming an apodizedvolume grating in the PP layer 510 of FIG. 5 are illustrated in FIG. 8with further reference to FIGS. 7A to 7C. A volume grating may beapodized by photoinducing (FIG. 8 ; 802) a dependence of the refractiveindex contrast Δn of a volume grating on a thickness coordinate z. Therefractive index variation is obtained by exposing a photopolymer layerto grating forming light.

The specific chemical process(es) utilized to achieve the requireddegree of apodization Δn(z) of a volume grating may depend on the orderof application of the apodization exposure relative to the gratingforming exposure. For example, in a first embodiment 811, theapodization exposure is applied before the grating forming exposure.This corresponds to FIG. 7A. The periodic refractive index variations inthe PP layer 510 may be formed due to a photoreactive agent of thephotopolymer being sensitive to illumination with the grating forminglight 704. The amplitude of the periodic refractive index variations maybe reduced due to the photoreactive agent being also sensitive toillumination with the apodization light 702. The apodization exposuremay selectively consume some of the Δn writing chemistry, i.e. thecapacity of the photopolymer to undergo a photochemically induced changeof the refractive index.

The writing chemistry does not necessarily need to be consumed, it couldalso be inhibited or otherwise have its overall sensitivity reduced,such that the refractive index variation Δn is less proximate theopposed surfaces of the PP layer 510 as compared to the value of Δn atthe center of the PP layer 510, e.g. at Z-coordinate of half thethickness t. In the first embodiment 811, the apodization wavelengthλ_(apodization) may be different than the grating wavelengthλ_(grating), and may even be outside of the wavelength range of thegrating forming light 704 altogether. For example, the apodizationwavelength λ_(apodization) may be selected to be within a strongabsorption band of the photoreactive agent of the PP layer 510.

In a second embodiment 812, the apodization exposure is appliedconcurrently with the grating forming exposure. This corresponds to thepreviously considered FIG. 7B. In the second embodiment 812, thephotopolymer recording material formulation may be modified toincorporate not only a photoreactive agent for forming the periodicrefractive index variations by a photoreaction (i.e.photopolymerization) to the grating forming light, but also anadditional component such as, for example, a photoinhibitor agent forimpeding the photoreaction when illuminated with the apodization light.The “inhibiting” exposure wavelength λ_(inhibit) may be different fromthe grating forming wavelength λ_(grating) In some variants of thesecond embodiment 812, the material formulation for the PP layer 510 maybe designed to be wavelength-orthogonal, in other words, the apodizationlight 702 at the wavelength λ_(inhibit) substantially does not activatethe photoreactive agent, i.e. does not induce polymerization, and thegrating forming light 704 at the grating forming wavelength λ_(grating)substantially does activate the photoinhibitor agent, i.e. does notinduce inhibition of the photopolymerization reaction. In some variants,the photochemical process and material composition are selected suchthat no other reactions may occur. The light exposure may besynchronized, and a relative irradiation may be varied to fine tune thedepth of refractive index modulation, Δn(z). Photoinhibitor additivematerials may undergo photolysis to produce a radical for polymerizationtermination. Non-limiting examples of such materials include butylnitrite, hexaarylbiimidazole, and tetraethylthiuram disulfide (TED).

In a third embodiment 813, the apodization exposure is applied after thegrating forming exposure, as illustrated in FIG. 7C. The purpose of theapodization exposure in this case is to locally induce a change therefractive index n and/or the refractive index variation Δn of theformed grating. The photopolymer material formulation may includephotoreactive groups whereby the photoreaction results in a change inrefractive index variation Δn, and/or photoreactive groups causingincrease or decrease of the refractive index n. This may be achieved bya reversible photopolymerization in such groups as azobenzene, stilbene,spiropyran, diarylethene; by photoelimination in diazo and/or azidogroups; an additional step of photopolymerization/crosslinkingindependent of the photopolymerization for the volume grating exposure;photoisomerization; and/or by photolocking. In some embodiments, theapodization exposure may initiate depolymerization/decrosslinkingreactions that allow high/low refractive index moieties in bright/darkfringes to diffuse and become spatially uniform. In some embodiments,isomerization reactions such as Photo-Fries rearrangement disclosed inOptical Materials 35 (2013) 2283-2289 incorporated herein by reference,and/or Calixarene isomerization disclosed in Bull. Chem. Soc. Jpn., 77,1415-1422 (2004)) incorporated herein by reference, may be used.

For any of the embodiments considered herein, the apodization wavelengthλ_(apodization) of the apodization light 702 may be selected for theapodization light 702 to be strongly absorbed by the recording materialof the PP layer 510. For example, no more than 5% of the optical powerof the apodization light 702 may be transmitted through the PP layer 510in some embodiments. The absorption of the apodization light 702 ensuresthat the refractive index contrast Δn is reduced only at the opposedsurfaces of the PP layer 510 where the exposure of the PP layer 510 tothe apodization light 702 is high. The apodization light 702 may beprovided for a short amount of time, e.g. less than 1 second. Theapodization exposure energy may be controlled with high precision andspatial uniformity, e.g. less than 5% non-uniformity of the apodizationlight 702 exposure. In embodiments where the apodization and exposurewavelengths are different, the PP layer 510 can be optimized forholographic recording with a low absorption at λ_(grating) and highabsorption at apodization wavelength λ_(apodization).

In some embodiments, the apodization light 702 exposure does not consumethe dynamic range of the PP layer 510 considerably, e.g. the dynamicrange reduction due to the apodization light 702 exposure may be lessthan 10-20%. For example and without limitation, the number of groupsphotopolymerized by the apodization light may be kept to a minimum whileproviding apodization of the refractive index contrast Δn. Theapodization light 702 may be provided immediately before, or within apre-defined time interval, of the intended exposure of the PP layer 510with the grating forming light 704.

The wavelength at which photoreactive groups absorb may be outside ofthe visible light spectrum, typically in ultraviolet wavelength range.In some embodiments, the photoreactive groups are located on thephotopolymer backbone. Photoinduced apodization methods disclosed hereinmay induce strong Δn modulation without significantly changing the baserefractive index, shrinkage properties, or other factors which mayaffect a Bragg reflection condition through the thickness of the PPlayer 510, before, during, or after the exposure(s). Otherwise, acomplementary method for modifying these may be used to obtain thedesired grating response.

In some embodiments, the grating apodization is chemically induced, or achemical apodization of the grating complement the photoinducedapodization considered above. An overall geometry of a chemicalapodization embodiment is presented in FIG. 9 , where chemically activelayers 920, or inhibitor layers, are provided on opposed parallel sidesof the PP layer 510. At least one active layer 920 may be provided. Thevolume gratings, e.g. VBGs or volume holograms, may be formed byilluminating the PP layer 510 with the grating forming light. Thegrating formation may be impeded by a reactive agent present in thechemically active layers 920, e.g. by controllably suppressing thephotopolymerization process to reduce an amplitude of the periodicrefractive index variations of the grating 520.

The amplitude of the periodic refractive index variations of the grating520 may be reduced using a variety of chemical processes. For example,after the grating forming photopolymerization, end groups of apolymerized photopolymer may be separated chemically and/orphotochemically, followed by a diffusion of the separated end groupsaway from their original locations, causing the formed grating to bewashed out. Referring to FIG. 10 , a photopolymer 1000 includes aphotopolymerizable group 1002 connected to an end group 1004 by anacid-cleavable linker group 1006. The end group 1004 may be a highrefractive index moiety, such that the local refractive index isdefined, at least in part, by the end group 1004. The reactive agent inthe chemically active layer(s) 920 (FIG. 9 ) may include an acid forseparating the end group 1004 (FIG. 10 ) from the photopolymerizablegroup 1002 by reacting with the acid-cleavable linker group 1006. Theend group 1004 may be separated from the correspondingphotopolymerizable group by application of the acid to at least one ofthe outer surfaces 521, 522 of the PP layer 510 (FIG. 9 ). The separatedend groups 1004 (FIG. 10 ) diffuse away after separation, therebyreducing the amplitude of the periodic refractive index variationsproximate the surface where the acid was applied. The acid may enter thePP layer 510 by diffusion from the chemically active layer(s) 920,and/or may be photochemically generated in the PP layer 510.

Turning to FIG. 11 , a general method 1100 of fabrication a gratingcoupler includes forming (1102) a plurality of volume gratings of anin-coupler or an out-coupler. The volume gratings may be apodized (1104)to achieve a desired spatial profile of the refractive index contrastΔn(z). Steps 1102 and 1104 may be performed in any order. The order ofperforming the steps 1102 and 1104 may depend on the chemistry used toachieve the grating apodization. In some embodiments, the forming step1102 includes exposing a photopolymer layer to grating forming light,and the apodization step 1104 includes exposing at least one surface ofthe photopolymer layer to apodization light, and/or exposing the atleast one surface to a reactive agent, as explained above with referenceto FIGS. 7A, 7B, 7C, FIG. 8 , and FIG. 9 .

Referring to FIG. 12 , an augmented reality (AR) near-eye display 1200includes a frame 1201 having a form factor of a pair of eyeglasses. Theframe 1201 supports, for each eye: a projector 1208 including a laserlight source described herein, a pupil-replicating lightguide 1210optically coupled to the projector 1208, an eye-tracking camera 1204,and plurality of illuminators 1206. The pupil-replicating lightguide1210 may include any of the grating-based in-couplers and/orout-couplers including the volume gratings as described herein. Theilluminators 1206 may be supported by the pupil-replicating lightguide1210 for illuminating an eyebox 1212. The projector 1208 provides a fanof light beams carrying an image in angular domain to be projected intoa user's eye. The pupil-replicating lightguide 1210 receives the fan oflight beams and provides multiple laterally offset parallel copies ofeach beam of the fan of light beams, thereby extending the projectedimage over the eyebox 1212.

The purpose of the eye-tracking cameras 1204 is to determine positionand/or orientation of both eyes of the user. Once the position andorientation of the user's eyes are known, a gaze convergence distanceand direction may be determined. The imagery displayed by the projectors1208 may be adjusted dynamically to account for the user's gaze, for abetter fidelity of immersion of the user into the displayed augmentedreality scenery, and/or to provide specific functions of interactionwith the augmented reality.

In operation, the illuminators 1206 illuminate the eyes at thecorresponding eyeboxes 1212, to enable the eye-tracking cameras toobtain the images of the eyes, as well as to provide referencereflections i.e. glints. The glints may function as reference points inthe captured eye image, facilitating the eye gazing directiondetermination by determining position of the eye pupil images relativeto the glints images. To avoid distracting the user with illuminatinglight, the latter may be made invisible to the user. For example,infrared light may be used to illuminate the eyeboxes 1212.

Embodiments of the present disclosure may include, or be implemented inconjunction with, an artificial reality system. An artificial realitysystem adjusts sensory information about outside world obtained throughthe senses such as visual information, audio, touch (somatosensation)information, acceleration, balance, etc., in some manner beforepresentation to a user. By way of non-limiting examples, artificialreality may include virtual reality (VR), augmented reality (AR), mixedreality (MR), hybrid reality, or some combination and/or derivativesthereof. Artificial reality content may include entirely generatedcontent or generated content combined with captured (e.g., real-world)content. The artificial reality content may include video, audio,somatic or haptic feedback, or some combination thereof. Any of thiscontent may be presented in a single channel or in multiple channels,such as in a stereo video that produces a three-dimensional effect tothe viewer.

Furthermore, in some embodiments, artificial reality may also beassociated with applications, products, accessories, services, or somecombination thereof, that are used to, for example, create content inartificial reality and/or are otherwise used in (e.g., performactivities in) artificial reality. The artificial reality system thatprovides the artificial reality content may be implemented on variousplatforms, including a wearable display such as an HMD connected to ahost computer system, a standalone HMD, a near-eye display having a formfactor of eyeglasses, a mobile device or computing system, or any otherhardware platform capable of providing artificial reality content to oneor more viewers.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. A method of fabrication of a grating coupler, themethod comprising: exposing a photopolymer layer having a thicknessbetween opposed first and second surfaces to grating forming light forforming periodic refractive index variations in the photopolymer layer;and exposing at least the first surface of the photopolymer layer to areactive agent for reducing an amplitude of the periodic refractiveindex variations proximate the first surface.
 2. The method of claim 1,wherein a photopolymer of the photopolymer layer comprises aphotopolymerizable group connected to an end group by an acid-cleavablelinker group.
 3. The method of claim 2, wherein a local refractive indexis defined, at least in part, by the end group.
 4. The method of claim3, wherein the reactive agent comprises an acid for separating the endgroup from the photopolymerizable group.
 5. The method of claim 4,wherein in operation, the end group diffuses away upon being separatedfrom the corresponding photopolymerizable group by application of theacid to the at least first surface, thereby reducing the amplitude ofthe periodic refractive index variations proximate the at least firstsurface.
 6. A grating coupler manufactured by: exposing a photopolymerlayer having a thickness between opposed first and second surfaces tograting forming light for forming periodic refractive index variationsin the photopolymer layer; and exposing at least the first surface ofthe photopolymer layer to a reactive agent for reducing an amplitude ofthe periodic refractive index variations proximate the first surface. 7.The grating coupler of claim 6, wherein a photopolymer of thephotopolymer layer comprises a photopolymerizable group connected to anend group by an acid-cleavable linker group.
 8. The grating coupler ofclaim 7, wherein a local refractive index is defined, at least in part,by the end group.
 9. The grating coupler of claim 8, wherein thereactive agent comprises an acid for separating the end group from thephotopolymerizable group.
 10. The grating coupler of claim 9, wherein inmanufacturing, the end group diffuses away upon being separated from thecorresponding photopolymerizable group by application of the acid to theat least first surface, thereby reducing the amplitude of the periodicrefractive index variations proximate the at least first surface.
 11. Adisplay device comprising: a projector for providing image light; and agrating coupler for at least one of in-coupling or out-coupling theimage light, the grating coupler manufactured by: exposing aphotopolymer layer having a thickness between opposed first and secondsurfaces to grating forming light for forming periodic refractive indexvariations in the photopolymer layer; and exposing at least the firstsurface of the photopolymer layer to a reactive agent for reducing anamplitude of the periodic refractive index variations proximate thefirst surface.
 12. The display device of claim 11, wherein aphotopolymer of the photopolymer layer comprises a photopolymerizablegroup connected to an end group by an acid-cleavable linker group. 13.The display device of claim 12, wherein a local refractive index isdefined, at least in part, by the end group.
 14. The display device ofclaim 13, wherein the reactive agent comprises an acid for separatingthe end group from the photopolymerizable group.
 15. The display deviceof claim 14, wherein in manufacturing, the end group diffuses away uponbeing separated from the corresponding photopolymerizable group byapplication of the acid to the at least first surface, thereby reducingthe amplitude of the periodic refractive index variations proximate theat least first surface.